Wind power converter typically utilizes AC-DC-AC conversion technology, wherein a voltage or current DC-link is employed.
FIG. 1 illustrates a theoretical topology of a conventional wind power converter with voltage DC-link. According to FIG. 1, the wind power converter 1 includes a wind generator 10, a wind power converter 11 and power grid 12. The wind power converter 11 converts the electrical power generated by the wind generator 10 and feed the converted electrical power to the power grid 12. The wind power converter 11 further includes a three-phase two-level rectifier 110, a DC-link capacitor 111 and a three-phase inverter 112. The three-phase two-level rectifier 110 comprises six diodes D1 to D6, and each of the diodes is connected with a power switch in anti-parallel. The three-phase inverter 112 includes six power switches S1 to S6. Theoretically, the value of the breakdown voltage of each of the diodes in the three-phase two-level rectifier or each of the power switches in the three-phase inverter 112 shall be at least above the value of the voltage across the DC-link capacitor 111. The higher is the value of the output voltage to the power grid desired, the higher is the value of the breakdown voltage is required. As with breakdown voltage increasing the semiconductor cost drastically increases, thus it incurs a higher cost.
From the perspective of engineering, the skilled person shall consider a compromise between the value of the breakdown voltage and the output voltage to the power grid as well as the feeding electrical power quality. FIG. 2 illustrates a conventional wind power converter with voltage DC-link, 3-level neural point clamp topology (3-level1 NPC). According to FIG. 2, the wind power converter 2 includes a wind generator 20, a wind power converter 21 and power grid 22. The wind power converter 21 converts the electrical power generated by the wind generator 20 and feed the converted electrical power to the power grid 22. The wind power converter 21 further includes a three-phase three-level rectifier 210, two DC-link capacitors connected in series 211 and a three-phase three-level inverter 212. The three-phase three-level converter 210 comprises twelve diodes D1 to D12, and each of the diodes is connected with a power switches in anti-parallel; additional diodes Da to Df are coupled as shown FIG. 2. The three-phase inverter 212 includes twelve power switches S1 to S12, and Dg to D1 are coupled as shown FIG. 2. By having such configuration, the value of the breakdown voltage for the diode or the power switch may be at least above the voltage across either of the DC-link capacitor. The required value of the breakdown voltage is reduced, but the number of the power semiconductor is increased, thus the cost reduction brought by the former is mitigated by the cost increase brought by the latter in some degree. If for higher voltage, the issue of semiconductor cost comes out likewise. In addition, the topology also becomes complex, increasing the coupling among the power devices in the wind power converter.